Control valve instrumentality

ABSTRACT

An open center directional control valve assembly having an unloading valve that diverts inlet flow to drain without passing through the entire assembly whenever all control valves are in neutral. When one or more of the control valves are operative, to actuate their associated motors at desired speeds, the unloading valve is automatically positioned by the pressure difference between inlet pressure and the highest motor pressure to deliver the necessary flow to operate the motors at the desired speeds while diverting the remainder of inlet flow to drain.

nite States atent 1 1 Ailshie et al.

[45] June 11, 1974 2,988,106 6/1961 Rue .1 91/462 X 3,447,567 6/1969 Tennis 137/59613 3, 02,104 8/1971 Stremple 137/59613 x Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or FirmEdward L. Brown, Jr.

[ ABSTRACT An open center directional control valve assembly having an unloading valve that diverts inlet flow to drain without passing through the entire assembly whenever all control valves are in neutral. When one or more of the control valves are operative, to actuate their associated motors at desired speeds, the unloading valve is automatically positioned by the pressure difference between inlet pressure and the highest motor pressure to deliver the necessary flow to operate the motors at the desired speeds while diverting the remainder of inlet flow to drain.

15 Claims, 6 Drawing Figures CONTROL VALVE INSTRUMENTALTTY This invention relates to hydraulic control valves for open center type systems.

Hydraulic systems of the open center type normally include an inexpensive, fixed displacement source of pressure fluid which supplies constant, maximum flow to control valves which direct a desired quantity of the fluid flow to a hydraulic motor to operate same at a selected speed. The pressure maintained in this system is just that which is necessary to overcome pressure losses in the system and operate the motor. Thus, an open center system has the desirable characteristic of operating at minimum pressure and requires instrumentality that is relatively simplified and economical in design.

Waste of power and heat build-up in open center systems arise from its continuous operation at maximum flow rates. In stand-by condition wherein no motors are being operated, the entire output flow from thesource is returned to drain or reservoir, and the power utilized in moving this high volume of fluid is completely dissipated into heat. During most operating conditions, the motor or motors will be operating at less than maximum speed. This is accomplished by the hydraulic control valves in the system which direct the desired quantity of flow to the motor while allowing the excess flow to return to reservoir. Again, movement of this excess flow is waste of power reducing overall efficiency of the system.

An open center hydraulic system for operating a plurality of motors conventionally includes a plurality. of manual directional control valves, one associated with each motor, which are contained within or assembled as a single unit or bank. Fluid flow from the source passes through a central bypass network of zig-zag passages extending through the bank to communicate with the reservoir. A manually shiftable member, usually a cannelured spool, in each of the control valves can be moved to partially or completely block flow through this central, open center network while directing the desired portion of flow to the motor associated with the shifted spool. Thus, during stand-by the full inlet flow from the source passes through the entire valve bank and across each of the valve spools, and a portion of the inlet flow normally passes through the open center network in many operating conditions. This causes substantial pressure losses to the inlet flow as it passes through the entire valve bank, and the resulting back pressure exerted on the source substantially increases the pressure at which the source must work just to force the fluid flow through the valve bank.

Upon shifting one of the spools to interrupt or meter flow through the open center network, strong forces resulting primarily from Bernoulli effects are impressed upon the spool. Characteristically therefore, open center valves require a high degree of effort in shifting their manually operated spools. Furthermore, interruption of the bypass flow in the open center network creates difflculty in precisely metering flow to the motor since the pressure drop across the spool at the bypass passages will vary in accordance with the load imposed on the motor which dictates the pressure level necessary to operate same. Also, the effort required to move the spool will also change erratically as the pressure varies. Control of motor operation in open center systems therefore is relatively erratic and imprecise because of the necessity of interrupting and metering the high volume bypass flow in the open center network.

The problems of efficiency loss, high spool effort and imprecise control in open center systems become extremely serious in larger hydraulic systems operating at higher rates of flow and pressure. As the application and uses of hydraulic instrumentality increase, thereby requiring greater flows and power, the above-described problems limit the usefulness of open-center systems.

As an alternative to open center systems, closed center systems have found limited acceptance in the industry in certain specific applications. Generally, closed center systems have a variable displacement source of pressure fluid that delivers only the flow required to operate the motor at the desired speed, i.e. zero low in stand-by conditions, but is maintained at high, maximum pressure at all times. The high pressure of closed center systems causes efficiency losses in excess of that of open center systems in most applications and detracts from the desirability of closed center systems even though the problems of high spool effort and erratic control are substantially reduced by eliminating the continuous bypass flow characteristic of the open center system. Further, the rather sophisticated and expensive instrumentality of closed center systems has prevented general acceptance of this arrangement.

It has been attempted to include in open center systems unloading valves which divert the excessive flow from the source back to reservoir without flowing through the entire valve bank to thereby reduce spool effort and improve control. Such attempts have required inclusion of relatively expensive and complicated circuitry which increases cost and substantially reduces reliability of the system. These arrangements also impress a relatively high stand-by pressure at the source, similar to closed center systems, and have been found generally impractical in use because of the inability to sense both the pressure at which the motors are operating as well as the position of the valve spool when in neutraL-stand-by condition.

It is a primary object of the present invention to provide control valve instrumentality for open center by draulic systems which provides a minimum stand-by power loss by diverting all flow to reservoir without passing through the control valves whenever the latter are in their neutral positions and without requiring a high stand-by pressure at the source, along with reducing the effort required to operate the manual spools and increasing the precision of control of the valves by eliminating the network of bypass passages by providing a pressure sensitive valve that automatically diverts the excess flow to reservoir without passing across the manually actuated spools.

Another object of the invention is to provide a pressure operated unloading valve upstream of a manual control valve which is operated by the pressure differential across a speed-selecting orifice created by the manual valve in such a manner to maintain a constant pressure drop across the orifice by diverting excess flow to reservoir before reaching the control valve, and wherein the'unloading valve is also sensitive to positioning of the manual valve in its neutral position to divert all flow directly to reservoir.

These features are accomplished by providing an unloading valve urged to a position unloading all flow to drain by pressure upstream of the orifice, and biased in an opposite direction by pressure downstream of the orifice when the manual valve is in an operating position. Upon shifting the manual valve to neutral, pressure is automatically relieved from one side of the unloading valve to allow it to move to its unloading position. More specifically, a pressure chamber at one end of the unloading valve is connected with the reservoir through a first pressure-transmitting pilot passage whenever the manual valve is in neutral, and is connected with pressure downstream of the orifice through a second pressure-transmitting pilot passage whenever the manual valve is shifted away from neutral.

Another object of the invention is to provide such hydraulic controls useful in a system having a plurality of manual control valves by arranging the first pilot passage in series with each of the control valves so that shifting of any one valve away from neutral will block communication of the unloading valve pressure chamber with drain, and arranging the second pilot passage in parallel with each of the control valves so that the highest pressure required by any of the valves will be transmitted to the pressure chamber to control the unloading valve. A corollary is to include means in the second pilot passage which substantially prevents cross-flow between motors therethrough to assure totally independent control of each motor.

A more particular object is to provide a restricted flow from the source directly to the pressure chamber of the unloading valve, and to include a relief valve connected to the pressure chamber to protect the entire system from overpressurization simply by relieving the relatively small flow from the source into the pressure chamber, thereby eliminating the need for a large high pressure relief valve in the system capable of dumping the entire volume of flow to reservoir in conditions of excessive pressure.

These and other important objects and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of preferred embodiments of the invention, when read in conjunction with the accompanying drawings, wherein:

FIGS. l-A and l-B are partially schematic representations of a hydraulic system constructed in accordance with the principles of the present invention, with the directional control valve and the pressure sensitive unloading valve being shown in cross-sectional elevation;

. there is illustrated the inlet portion section and a control valve section 12 which are conventionally intersecured into a single, open center stack valve assembly. Inlet section 10 includes a housing 14 having an inlet port and passage 16 supplied with pressure fluid from a source 18 that is normally a fixed, positive displacement pump. Section 10 also has a longitudinal bore 20 closed at both ends by plugs 22 and 24, and a pair of exhaust passages 26 intercepting bore 20 adjacent inlet passage 16 are connected with a low pressure reservoir 28 as shown schematically.

Shiftably disposed within bore 20 is a pressure responsive unloading valve spool 30 which defines closed pressure chambers 32 and 34 at its opposite ends. Spool 30 blocks communication between exhaust passages 26 and inlet passage 16 when shifted leftwardly, as illustrated, under the urgings of a biasing spring 36, and has grooves 44 for interconnecting the inlet passage with passages 26 upon being shifted rightwardly from the position illustrated. Internal passages 38 and 40 and cross bores 42 in spool 30 define first and second ducts allowing a highly restricted flow from the source and inlet passage 16 to the pressure chambers 32 and 34 respectively.

Control valve section 12 includes a housing 13 having a longitudinal bore 46 which is intercepted at spaced locations by a motor port passage 48 and an inlet passage 50. The valve illustrated is of the double acting type having a second motor port passage-52 intercepting the bore 46 adjacent a different leg of U- shaped inlet passage 50. The motor port passages are connected to supply and exhaust fluid from opposite sides of a double acting hydraulic motor 54 that has a load 56 imposed thereon. Housing 13 also has a duct 58 intercepting bore 46, as well as a pair of adjacent ducts 60 and 62 that can communicate through bore 46. Duct 60 is connected with reservoir as shown schematically.

A manually shiftable control member in the form of a cannelured spool 64 is movably disposed in bore 46 and is biased to a neutral, stand-by position illustrated by a conventional spring centering assembly 66. Spool 64 has internal central bores 68 and 70 that respectively connect associated cross bores 72 and 74 which terminate adjacent the motor port passages 48 and-52 as well as on opposite sides of duct 58. In neutral, passages 68 and 70 are isolated both from motor passages 48 52 and from duct 58.

Upon securng inlet section 10 to control valve sec tion 12, direct communication between inlet passage 16 with a transverse passage 76 in control valve 12 is provided, as illustrated by phantom line 78. Pressure in passage 76 acts upon one-way check valve 80 to open same to direct pressure fluid into U-shaped inlet passage 50 since the effective seat area a" of the check valve is less than the effective plunger area b urging it in the opposite direction. Conventionally, whenever pressure in passage 50 exceeds that in passage 78, valve 80 will close to prevent reverse flow. Drain passages 82 and 84 at opposite ends of both sections respectively register with each other, and both drain passages are connected to the reservoir. A passage in the inlet section housing 14, depicted by line 86, registers with duct 62 in the control valve housing as shown by dashed line 88. A duct in the inlet housing shown by line connects with passage 86 and thus pressure chamber 34, and duct 90 registers with duct 58 in the control valve. As can be seen in FIG. 3, duct 90 extends transversely through valve section housing 13 with a vertical passage 92 connecting passage 92 and duct 58. Disposed in duct 90 is a one-way check valve and restriction 102 which allows one-way, restricted flow from pressure chamber 34 into passage 90 and the duct 58 in the control valve.

A high pressure relief valve assembly 94, mounted within plug 24, communicates with pressure chamber 34 by orifice 96, and connects with drain passage 84 via passage 98. Normally, the relief valve assembly blocks flow from orifice 96 to passage 98, but interconnects same to relieve pressure in chamber 34 whenever pressure therein exceeds a preset level. Relief valve assembly 94 is conventional in structure and operation and will not be described in detail.

In operation, a continuous relatively constant flow of pressurized fluid passes from source 18 into inlet passage 16 and through line 78 into passage 76 and across lift check valve 80 into inlet passage 50 of the control valve. When control spool member 64 is in its neutral position illustrated, the inlet flow is blocked at bore 46. A first pilot passage defined by line 88 and ducts 62, 60 interconnects the unloading spool pressure chamber 34 with the low pressure reservoir by way of groove 104 in control spool member 64. A second pilot passage defined by passages 90, 92, duct 58 and internal bores 68, 70 as well as their associated cross bores 72, 74 also communicates with pressure chamber 34 by line 86. In its neutral position, control pool 64 blocks flow through this second pilot passage by virtue of the positions of cross bores 72 and 74.

Accordingly, interconnection of unloading valve pressure chamber 34 with reservoir by way of the first pilot passage, including ducts 60 and 62,'relieves pressure so that once inlet pressure in passage 16 and the other pressure chamber 34 builds to a level sufficient to overcome the bias of spring 36, unloading spool 30 shifts rightwardly to allow free communication of inlet passage 16 with exhaust passages 26. In this unloading position of spool 30 all flow from the source is diverted directly back to reservoir without passing through control valve section 12. A minimum back pressure is thus maintained in stand-by condition, only sufficient pressure to overcome the bias of spring 36. To assure minimum pressure in chamber 34, the cross bore 42 is sized to present a restriction to fluid flow into chamber 34 so that fluid flow exhausting out of chamber 34 through the first pilot passage is sufficient to drop pressure in chamber 34 to that of the reservoir.

Upon shifting control spool member 64 leftwardly, an orifice of communication which is variable in size dependent upon the travel of spool 64, is established between inlet passage 50 and motor passage 52 at bore 46 through groove 108 in spool 64. The motor 54 is operated leftwardly, and fluid displaced out the opposite side of the motor returns into passage 48 to exhaust passage 82 through groove 106. In certain designs it may be desirable to form the variable size orifice in the exhaust flow at groove 106 rather than in the inlet flow. In either instance the operation of the present invention will proceed as described below, and description of the variable orifice as being positioned in the inlet flow to the motor is to be considered as also including an embodiment wherein orifice of discharge flow from the motor actually accomplishes speed control thereof.

In this operating position spool 64 blocks flow between ducts 62 and 60 to isolate chamber 34 from the reservoir. Internal bore 70 connects duct 58 with pressurized motor passage 52 so that flow from chamber 34 through the second pilot passage is directed into motor passage 52 downstream of the variable orifice created by spool 64. Accordingly, the lower pressure downstream of the orifice is maintained in chamber 34 in opposition to the higher inlet pressure in chamber 32.

This pressure differential imposed at opposite endsof unloading spool 30 automatically shifts the latter to a position allowing a certain rate of flow from inlet passage 16 to exhaust passages 26. Spool 30 will reach an effective position allowing a certain rate of flow to inlet passage 50 of the control valve which will produce a pressure drop across the orifice formed by spool 64 that will be balanced by spring 36. The rate of flow passing into control valve 12 will bethat required to operate the motor at the speed selected by the manual positioning of spool 64. A constant pressure drop across the spool-defined orifice results, equivalent to the force of spring 36. This constant, relatively low pressure drop creates small, and in the least, constant longitudinal forces on spool 64 so that reliable manual control of the spool is possible even in larger sized control valves. Because the excess flow from the inlet does not cross valve spool 64, but is rather bypassed directly to drain passage 26, the effort to move spool 64 remains small regardless of the position thereof.

Relief valve assembly 94 opens whenever pressure in chamber 34 exceeds a predetermined level. Unloading spool 30 in response shifts rightwardly to its unloading position diverting all low to passages 26 to reduce system pressure to lower levels. Relief valve 94 may be small, compact and economical in design as it only need carry the restricted flow into chamber 34 via cross bore 42 in order to reduce chamber pressure. Thus, the expense and bulkiness of a system relief valve capable of carrying the entire flow from source 18 is eliminated. Check valve prevents reverse flow from the motor through the second pilot passage and across relief valve 94 when the latter is'open, and thereby minimizes the volume of fluid required to flow through the relief valve as well as preventing undesirable motor movement when the relief valve opens.

Spool 64 is shiftable further leftwardly to a float position wherein spool land interrupts flow between inlet passage 50 and motor passage 52, while spool groove 112 connects motor passage 52 with drain passage 84. The other motor passage 48 remains connected with drain passage 82 through groove 106 so that the motor may float" free in response to loads imposed thereon. In the float position duct 58 is reclosed by spool 64 to block the second pilot passage, while spool groove 114 reconnects ducts 60 and 62 to drain pressure from chamber 34. Accordingly, unloading spool 30 shifts rightwardly to its unloading position bypassing all fluid flow directly to the reservoir.

Spool 64 can be shifted rightwardly to pressurize motor passage 48 while connecting motor passage 52 with drain passage 84. This movement of spool 64 connects the pressure chamber 34 with pressurized passage 52 through internal bore 68 in spool 64, while communication between ducts 60 and 62 is interrupted. Accordingly, operation in this position driving the motor in an opposite direction is the same as occurs when spool 64 is shifted to pressurize motor passage 52.

FIG. 5 schematically illustrates an embodiment of the invention which includes not only the unloading valve section 10 and control valve section 12 of FIGS. l-A and 1-3, but also additional control valve sections 12a and 12b which are substantially identical in construction to the control valve 12 described above in detail. Reference numerals with a and b suffixes denote structure in valvesl2a and 12b identical to the control valve 12 of FIG. l-A. The respective control spools of valves 12a and 12b are shiftable oppositely from the neutral positions schematically illustrated to positions metering fluid flow to opposite ends of associated motors 54a and 54b.

The transverse passage 76 of valve 12 registers with similar transverse passages in valves 12a and 12b so that the inlet passages 50, 50a and 50b connect in hydraulicallyparallel relationship with inlet passage 16 of the inlet section. Associated lift check valves 80a and 80b allow only one-way flow in the associated inlet 1385-. sage toward the motors. The exhaust passages 82, 84 include identical portions in the additional valves to extend transversely therethrough, passages 82, 84 being denoted by a single return line in FIG. 5. Duct 60 of valve 12 connects with duct 62a of valve 12a, and duct 60a connects with duct 62b of valve 12b, with the final duct 60b communicating with the reservoir such as by connection with exhaust passages 82, 84. Accordingly, the first pilot passages of the control valves are connected in hydraulic series arrangement intercepting each of the control valve spool members. With all spool members in neutral, the pressure chamber 34 of unloading valve communicates with the reservoir through the first pilot passage as shown. Upon shifting any one of the valve spool members away from neutral, flow between the associated ducts 62, 60 or 62a, 60a or 62b, 60b is interrupted to isolate the firstpilot passage and chamber 34 from the reservoir.

.Valves 12a and 1222 have transverse ducts 90a and 9012 that communicate with duct 90 of valve 12 to define a common duct connected with chamber 34, so that the associated vertical passages 92a and 92b and ducts 58a, 58b are connected in hydraulically parallel relationship with chamber 34. In neutral these second pilot passages aredead-ended by virtue of blockage of the respective ducts 58, 58a, 58b by the spool member of the control valves. The parallel connection of these second pilot passages allows the pressure downstream of the spool-defined orifice to be transmitted to chamber 34. upon movement of any of the spool members away from neutral. Each spool has the internal bores 68-74 of spool 64 in FIG. l-A so that the pressurized motor passage of each valve can be connected with chamber 34.

Upon simultaneously operating two or more of the control valves, both or all the pressurized motor passages connect with chamber 34 through the parallelarranged second pilot passages, so that the highest pressure existing at the motors being actuated is transmitted to chamber 34. Accordingly, thespool of the unloading valve will be positioned to divert unnecessary flow to drain while allowing delivery of flow necessary to the control valves to operate both motors at the selected speed, while maintaining pressure at the source at the level of that required by themotor operating at the highest pressure plus the pressure differential maintained across the spool member metering flow to that motor. I

A check valve 114 allowing only severely restricted flow through restriction 116 is interposed in passage 92 of the second pilot passage. Similar check valves 114a, 1141) and restrictions 116a, ll6b are disposed in passage 92a, 92b. One embodiment of check valve 114 illustrated in FIGS. 3 and 4 comprises a hexagonally shaped elementmovably carried in passage 92. Flow from passage toward duct 58 forces element 114 downwardly so that the lower face thereof seats on an annular shoulder 118 of passage 92, thereby allowing flow in this direction only through the central orifice restriction 1 16 of element 114. Flow in the opposite direction in passage 92 shifts element 114 upwardly permitting relatively free flow around the periphery of element 114 into passage 90.

The check valve elements I14, 114a, l14b substantially prevent crossflow between motor passages of different control valves through the second pilot passages when more than one control valve member are in operating positions. Only through one of the orifice restrictions 116, 116a, 1161) is such crossflow permitted. Yet, relatively free flow in the opposite directions assures that chamber 34 will be maintained at the highest pressure existing at any of the motors being actuated. The size of orifice cross bore 72 relative to orifices l 16 allow sufficient exhaust flow from chamber 34 when one of the control valves is in an operating position to maintain a pressure in chamber 34 indicative of that of the motor.

Operation of the FIG. 5 system is essentially the same as above described with respect to FIGS. l--A, l-B. The series arrangement of the first pilot passages allows chamber 34 to sense the neutral positioning of all the control valve spool members, while the parallel arrangement of the second pilot passages transmits the highest motor pressure to chamber 34. Relief valve 94 operates as previously described to limit systempressure. With all valves in neutral, the entire output flow from source 18 is diverted back to reservoir with only the small volume of pilot flow in the first pilot passage flowing through the whole valve bank. The unloading valve senses the total volume of flow required by one or more of the motors, and allows this volume to flow to the control valves while diverting the remainder to reservoir.

The detailed description of preferred embodiments set forth above is exemplary in nature and is not to be considered as limiting to the scope and spirit of the invention as set forth in the accompanying claims.

Having described the inventionwith sufficient clarity that those skilled in the art may make and use it, what is claimed as new and desired to be secured by letters Patent is:

1. In a hydraulic system having a source of pressure fluid, a low pressure reservoir, and a hydraulic motor connected with said source:

an unloading valve communicating with said source and said reservoir to allow direct flow therebetween bypassing said motor, said unloading valve having a bore therein closed at both ends;

a shiftable spool movably disposed in said bore for controlling said bypass flow from the source to reservoir and defining pressure chambers at opposite ends of said bore, said unloading valve having a duct providing flow from said source to one of said chambers whereby pressure therein urges said spool toward an unloading position connecting said source with the reservoir;

biasing means disposed in the other of said pressure chambers for urging said spool toward a position variably blocking communication between said source and said reservoir and thereby adjusting the rate of flow from said source to said motor;

a directional control valve interposed in the path of fluid flow from said source to said motor downstream of said unloading valve and communicating with the reservoir, said control valve having a first pilot passage connectable with the reservoir and a second pilot passage connectable with the motor, said first and second pilot passages communicating with said other pressure chamber of the unloading valve; and

a shiftable member mounted in said control valve for controlling communication between said source and said motor and intercepting said first and second pilot passages, said member having a neutral position blocking flow from said source to said motor and blocking communication between said second pilot passage and said motor while connecting said first pilot passage with the reservoir to thereby maintain low pressure in said other pressure chamber and allow pressure in said one pressure chamber to overcome the urgings of said biasing means and shift said spool to said unloading position thereof,

said member being selectively shiftable from said neutral position to an operating position establishing a variable size orifice of communication between said source and said motor to select speed of operation of the latter, said member in said operating position blocking communication between said first pilot passage and the reservoir while connecting said second pilot passage with said motor downstream of said orifice to transmit the pressure existing downstream of the orifice to said other pressure chamber to assist said biasing means in urging said spool to said variably blocking position, whereby said spool is positioned to maintain a substantially constant pressure differential across said variable size orifice and allow delivery of fluid to said control valve and motor at a rate operating said motor at said selected speed.

2. A hydraulic system as set forth in claim 1, wherein is provided a plurality of said hydraulic motors connected in parallel with said source downstream of said spool and a plurality of said directional control valves and corresponding shiftable members for controlling operation of said motors, said first pilot passages of said control valves being connected in series whereby shifting of any of said members away from its neutral position blocks communication of said other pressure chamber with the reservoir through said first pilot passages, said second pilot passages of said control valves being connected with said other pressure chamber in parallel arrangement whereby the highest pressure existing at said motors is transmitted by said second pilot passages to said other pressure chamber.

3. A hydraulic system as set forth in claim 2, wherein is provided a check valve in each of said second pilot passages allowing relatively free fluid flow from the associated motor to said other pressure chamber while severely restricting fluid flow through said second pilot passage in the opposite direction to reduce direct fluid flow between said motors through said second pilot passages.

4. A hydraulic system as set forth in claim 3, wherein is provided a common duct connected with said other pressure chamber, said second pilot passages being connected in parallel with said common duct, each of said check valves comprising a hexagonal element having an orifice passage therethrough and shiftably disposed in the corresponding second pilot passage to allow relatively free flow from said motor to said common ductpast the periphery of said hexagonal element while permitting flow from said common duct to said motor only through said orifice passage.

5. A hydraulic system as set forth in claim 1, wherein said unloading valve has a second duct connecting said source with said other pressure chamber, and means in said second duct for restricting flow therethrough whereby low pressure is maintained in said other pressure chamber when said control member is in its neutral position and pressure at said motor is maintained in said other chamber when said control member is in its operation position.

6. A hydraulic system as set forth in claim 5, wherein is included a high pressure relief valve communicating with said other pressure chamber and the reservoir, said relief valve being operable to open and relieve pressure from said other chamber to allow said spool to move to said unloading position when pressure in said other chamber exceeds a predetermined level.

7. A hydraulic system as set forth in claim 6, wherein is provided a check valve between said second pilot passage and said other chamber for preventing fluid flow from said motor to said other pressure chamber while allowing pressure at said motor to be transmitted to said other chamber.

8. A hydraulic system as set forth in claim 5, wherein said first mentioned duct and said second duct are separate internal passages in said spool respectively connecting said one and said other pressure chambers with said source. v

9. A hydraulic system as set forth in claim 1, wherein said control valve includes a port communicating with said motor, said second pilot passage including a duct in said control valve communicating with said other pressure chamber and intercepted by said control member, said second-pilot passage further including an internal passage in said control member which registers with said motor port and said duct in the control valve upon shifting said member to its operating position.

10. A hydraulic system as set forth in claim 9, wherein said control valve includes a pair of ports communicating with opposite sides of said motor, said control member being shiftable in opposite directions away from neutral to establish variable size orifices of communication between said source and said motor ports,

said second pilot passage including a pair of said internal passages in said control member respectively registering with said pair of motor ports as said member is shifted in opposite directions away from neutral to connect said other pressure chamber with said motor port that is communicating with said source.

11. A control valve for a hydraulic system including a fluid actuated motor, a fixed displacement source of pressure fluid, and a reservoir, said control valve comprising:

a housing having first and second spaced longitudinal bores, a generally transverse inlet passage connected to said source and sequentially intercepting said first and second bores, an exhaust passage communicating with the reservoir and intercepting said first bore adjacent said inlet passage, a motor passage connected with said motor and intercepting said second bore adjacent said inlet passage, and first and second pilot passages intercepted by said second bore, said first passage being connected with the reservoir;

a pressure responsive spool disposed in said first bore and shiftable to variably restrict fluid flow from said inlet passage to said exhaust passage and thereby vary the rate of flow delivered through said inlet passage to said second bore;

means for supplying pressure from said source to one end of said pressure responsive spool to urge the latter to an unloading position permitting free fluid flow from said inlet passage to said exhaust passage;

biasing means acting against the opposite end of said pressure responsive spool to urge the latter towards positions variably metering flow from the inlet to the exhaust passages;

a manually shiftable control spool in said second bore having a neutral position blocking said inlet passage while connecting said first pilot passage with the reservoir, and an operating position establishing a variable orifice of communication between I said inlet passage and said motor passage to control speed of operation of the motor while connecting said second pilot passage with said motor passage downstream of said variable orifice and blocking communication of said first pilot passage with the reservoir; and

means connecting said first and second pilot passages with said opposite end of the pressure responsive spool whereby to maintain low pressure of the rese'rvoir at said opposite end when said manual control spool is in its neutral position and to transmit pressure from said motor passage downstream of said variable orifice to said opposite end to assist said biasing means in moving said pressure responsive spool to said variable metering positions when said manual control is in its operating position to maintain a substantially constant pressure differential across said variable orifice.

12. A control valve as set forth in claim 11, wherein said housing includes a third longitudinal bore intercepting said inlet passage in hydraulically parallel arrangement with said second bore, and another motor passage intercepting said third bore adjacent said inlet passage, said first pilot passage intercepting said second and third bores in series arrangement, said second pilot passage intercepting said second and third bores in parallel arrangement; there being another manual control spool in said third bore for controlling flow from said inlet passage to said another motor passage, said another control spool being operable to block said first pilot passage in a neutral position and to connect said second pilot passage and the inlet passage with said another motor passage in an operating position.

13. A control valve as set forth in claim 12, wherein is included means in said second pilot passage for preventing fluid flow therethrough between said motor passages while transmitting the higher of the pressures in said motor passages to said opposite end of the pressure responsive spool.

14. A control valve as set forth in claim 11, wherein said housing includes a second motor port passage intercepting said second bore and communicating with said motor, whereby upon shifting said manual control spool in opposite directions away from neutral said firstmentioned and said second motor passages are respectively connected with said inlet passage, said second pilot passage being arranged to connect said opposite end of the pressure responsive spool only with the one of said first and second motor passages which is communicating with said inlet passage as said control spool is moved away from neutral.

15. A control valve as set forth-in claim 14, wherein said manual control spool is shiftablelto a float position interconnecting both said first and second motor port passages with reservoir while blocking flow from said source to said motor, said manual control spool in said float position connecting said first pilot passage with y the reservoir and blocking flow through said second pilot passage. 

1. In a hydraulic system having a source of pressure fluid, a low pressure reservoir, and a hydraulic motor connected with said source: an unloading valve communicating with said source and said reservoir to allow direct flow therebetween bypassing said motor, said unloading valve having a bore therein closed at both ends; a shiftable spool movably disposed in said bore for controlling said bypass flow from the source to reservoir and defining pressure chambers at opposite ends of said bore, said unloading valve having a duct providing flow from said source to one of said chambers whereby pressure therein urges said spool toward an unloading position connecting said source with the reservoir; biasing means disposed in the other of said pressure chambers for urging said spool toward a position variably blocking communication between said source and said reservoir and thereby adjusting the rate of flow from said source to said motor; a directional control valve interposed in the path of fluid flow from said source to said motor downstream of said unloading valve and communicating with the reservoir, said control valve having a first pilot passage connectable with the reservoir and a second pilot passage connectable with the motor, said first and second pilot passages communicating with said other pressure chamber of the unloading valve; and a shiftable member mounted in said control valve for controlling communication between said source and said motor and intercepting said first and second pilot passages, said member having a neutral position blocking flow from said source to said motor and blocking communication between said second pilot passage and said motor while connecting said first pilot passage with the reservoir to thereby maintain low pressure in said other pressure chamber and allow pressure in said one pressure chamber to overcome the urgings of said biasing means and shift said spool to said unloading position thereof, said member being selectively shiftable from said neutral position to an operating position establishing a variable size orifice of communication between said source and said motor to select speed of operation of the latter, said member in said operating position blocking communication between said first pilot passage and the reservoir while connecting said second pilot passage with said motor downstream of said orifice to transmit the pressure existing downstream of the orifice to said other pressure chamber to assist said biasing means in urging said spool to said variably blocking position, whereby said spool is positioned to maintain a substantially constant pressure differential across said variable size orifice and allow delivery of fluid to said control valve and motor at a rate operating said motor at said selected speed.
 2. A hydraulic system as set forth in claim 1, wherein is provided a plurality of said hydraulic motors connected in parallel with said source downstream of said spool and a plurality of said directional control valves and corresponding shiftable members for controlling operation of said motors, said first pilot passages of said control valves being connected in series whereby shifting of any of said members away from its neutral position blocks communication of said other pressure chamber with the reservoir through said first pilot passages, said second pilot passages of said control valves being connected with said other pressure chamber in parallel arrangement whereby the highest pressure existing at said motors is transmitted by said second pilot passages to said other pressure chamber.
 3. A hydraulic system as set forth in claim 2, wherein is provided a check valve in each of said second pilot passages allowing relatively free fluid flow from the assoCiated motor to said other pressure chamber while severely restricting fluid flow through said second pilot passage in the opposite direction to reduce direct fluid flow between said motors through said second pilot passages.
 4. A hydraulic system as set forth in claim 3, wherein is provided a common duct connected with said other pressure chamber, said second pilot passages being connected in parallel with said common duct, each of said check valves comprising a hexagonal element having an orifice passage therethrough and shiftably disposed in the corresponding second pilot passage to allow relatively free flow from said motor to said common duct past the periphery of said hexagonal element while permitting flow from said common duct to said motor only through said orifice passage.
 5. A hydraulic system as set forth in claim 1, wherein said unloading valve has a second duct connecting said source with said other pressure chamber, and means in said second duct for restricting flow therethrough whereby low pressure is maintained in said other pressure chamber when said control member is in its neutral position and pressure at said motor is maintained in said other chamber when said control member is in its operation position.
 6. A hydraulic system as set forth in claim 5, wherein is included a high pressure relief valve communicating with said other pressure chamber and the reservoir, said relief valve being operable to open and relieve pressure from said other chamber to allow said spool to move to said unloading position when pressure in said other chamber exceeds a predetermined level.
 7. A hydraulic system as set forth in claim 6, wherein is provided a check valve between said second pilot passage and said other chamber for preventing fluid flow from said motor to said other pressure chamber while allowing pressure at said motor to be transmitted to said other chamber.
 8. A hydraulic system as set forth in claim 5, wherein said first mentioned duct and said second duct are separate internal passages in said spool respectively connecting said one and said other pressure chambers with said source.
 9. A hydraulic system as set forth in claim 1, wherein said control valve includes a port communicating with said motor, said second pilot passage including a duct in said control valve communicating with said other pressure chamber and intercepted by said control member, said second pilot passage further including an internal passage in said control member which registers with said motor port and said duct in the control valve upon shifting said member to its operating position.
 10. A hydraulic system as set forth in claim 9, wherein said control valve includes a pair of ports communicating with opposite sides of said motor, said control member being shiftable in opposite directions away from neutral to establish variable size orifices of communication between said source and said motor ports, said second pilot passage including a pair of said internal passages in said control member respectively registering with said pair of motor ports as said member is shifted in opposite directions away from neutral to connect said other pressure chamber with said motor port that is communicating with said source.
 11. A control valve for a hydraulic system including a fluid actuated motor, a fixed displacement source of pressure fluid, and a reservoir, said control valve comprising: a housing having first and second spaced longitudinal bores, a generally transverse inlet passage connected to said source and sequentially intercepting said first and second bores, an exhaust passage communicating with the reservoir and intercepting said first bore adjacent said inlet passage, a motor passage connected with said motor and intercepting said second bore adjacent said inlet passage, and first and second pilot passages intercepted by said second bore, said first passage being connected with the reservoir; a pressure responsive spool disposed in said first bore and shIftable to variably restrict fluid flow from said inlet passage to said exhaust passage and thereby vary the rate of flow delivered through said inlet passage to said second bore; means for supplying pressure from said source to one end of said pressure responsive spool to urge the latter to an unloading position permitting free fluid flow from said inlet passage to said exhaust passage; biasing means acting against the opposite end of said pressure responsive spool to urge the latter towards positions variably metering flow from the inlet to the exhaust passages; a manually shiftable control spool in said second bore having a neutral position blocking said inlet passage while connecting said first pilot passage with the reservoir, and an operating position establishing a variable orifice of communication between said inlet passage and said motor passage to control speed of operation of the motor while connecting said second pilot passage with said motor passage downstream of said variable orifice and blocking communication of said first pilot passage with the reservoir; and means connecting said first and second pilot passages with said opposite end of the pressure responsive spool whereby to maintain low pressure of the reservoir at said opposite end when said manual control spool is in its neutral position and to transmit pressure from said motor passage downstream of said variable orifice to said opposite end to assist said biasing means in moving said pressure responsive spool to said variable metering positions when said manual control is in its operating position to maintain a substantially constant pressure differential across said variable orifice.
 12. A control valve as set forth in claim 11, wherein said housing includes a third longitudinal bore intercepting said inlet passage in hydraulically parallel arrangement with said second bore, and another motor passage intercepting said third bore adjacent said inlet passage, said first pilot passage intercepting said second and third bores in series arrangement, said second pilot passage intercepting said second and third bores in parallel arrangement; there being another manual control spool in said third bore for controlling flow from said inlet passage to said another motor passage, said another control spool being operable to block said first pilot passage in a neutral position and to connect said second pilot passage and the inlet passage with said another motor passage in an operating position.
 13. A control valve as set forth in claim 12, wherein is included means in said second pilot passage for preventing fluid flow therethrough between said motor passages while transmitting the higher of the pressures in said motor passages to said opposite end of the pressure responsive spool.
 14. A control valve as set forth in claim 11, wherein said housing includes a second motor port passage intercepting said second bore and communicating with said motor, whereby upon shifting said manual control spool in opposite directions away from neutral said firstmentioned and said second motor passages are respectively connected with said inlet passage, said second pilot passage being arranged to connect said opposite end of the pressure responsive spool only with the one of said first and second motor passages which is communicating with said inlet passage as said control spool is moved away from neutral.
 15. A control valve as set forth in claim 14, wherein said manual control spool is shiftable to a float position interconnecting both said first and second motor port passages with reservoir while blocking flow from said source to said motor, said manual control spool in said float position connecting said first pilot passage with the reservoir and blocking flow through said second pilot passage. 